5-1 Development of the Periodic Table

Why have a table? Chemists developed the Periodic Table to help organize and classify the elements.

J.W. Dobereiner Early 1800’s Observed elements could be classified into sets of three –Called Triads –Ex. Li, Na, K –Ex. Ca, Sr, Ba –Cl, Br, I

Elements within a triad have similar chemical properties. –In each triad, the properties of the middle element are close to the averages of the properties of the 1 st and 3 rd element. –Ca Sr Ba ( ) ÷ 2 =

J.A.R Newlands 1865 By this time, 62 elements were known. He observed that when elements were arranged in order of increasing atomic mass, the properties of the 8 th element were like the 1 st, 9 th –Law of Octaves

Dmitri Mendeleev 1869 Like Newlands, he saw a periodic repitition of properties when elements were arranged by increasing atomic mass –He developed the first periodic table Elements in the same column have similar properties

He also knew that not all elements were known and left gaps in this table for these undiscovered elements –He predicted the properties of the undiscovered elements, and it turns out he was correct. Example: Ekasilicon  Germanium

H.G.J. Moseley Remember Mendeleev organized his periodic table in order of increasing atomic mass. –This was not quite correct, the elements are arranged in order of increasing atomic number.

Periodic Law – When elements are arranged in order of increasing atomic number, their physical and chemical properties show in a periodic pattern.